1. Field of the Invention
The present invention relates generally to electro-mechanical actuators and particularly to piezoelectric actuators used to create high forces through small displacements for applications such as bending mirrors in laser optics systems.
2. Description of the Prior Art
The problem of maintaining a focused laser beam while the beam is traveling through a medium such as air has produced substantial difficulty even though advances in laser technology permit the projection of a laser beam over great distances. The energy concentration of a laser beam is higher in the central portion of the cross-sectional area than at the periphery; therefore, a laser beam heats the medium through which the beam travels more at the center of the beam than at the periphery thereof. Heating a gaseous medium, such as air, changes the index of refraction; thus the index of refraction at the center of the beam differs from that at the edges of the beam. The variation in refractive index of the medium causes the beam to refract toward the cooler gas regions, a condition known as blooming, which destroys the previous sharp focus of the beam.
Vibrations and distortions in the laser system optical elements also degrade beam sharpness. While the use of specially vibration-free stabilized optical benches and tables avoids the vibration problem in laboratory environments, the effects of vibrations become highly critical in applications of laser systems outside the laboratory. One solution to the problem of poorly focused laser beams is to bend or distort the reflecting surface of the laser apparatus to modify the optics and, thereby, compensate for undesired distortion to focus the beam at a predetermined location. To achieve focusing through mirror bending, it is necessary to adjust several surface bending points as described in "Phase Compensation for Thermal Blooming" by Lee C. Bradley and Ian Hermann in Applied Optics, Vol. 13, No. 2, p332.
Prior art methods of bending laser mirrors have deficiencies which limit the utility thereof in practical laser applications. Two of the most common deficiencies in the prior art are a failure of the mirror bending apparatus to achieve bending rapidly enough to compensate for vibration distortions, and the requirement of a very high voltage power source for supplying the necessary driving power. Although described with reference to laser technology, the foregoing deficiencies are common in any actuator application to a load which requires a high force with small displacement in a very short time interval.
Some prior art devices for bending laser mirrors include piezoelectric actuators incorporating single crystals and monolithic devices, which have exhibited both of the hereinabove mentioned deficiencies. A piezoelectric material produces a potential difference in response to the application of pressure thereto; and conversely, the application of a potential difference to a piezoelectric material produces a change in the physical dimensions thereof and, therefore, a force in the direction of the electric field. Single piezoelectric crystals and polycrystalline single body monolithic piezoelectric devices are unable to produce forces and displacements having magnitudes sufficient to bend laser mirrors with a response time to compensate for vibrational distortions. In addition, prior art systems using piezoelectric actuators to produce forces sufficient to bend laser mirrors have required excessively high power supply voltages, which necessitate the use of circuit components having substantial size and weight, which, along with the costs involved, have limited the utility of such devices to laboratory applications. Other prior art mirror bending methods are also expensive with response times of undesirably long duration and have the additional problem of being unable to accurately provide the high force required to bend the laser mirror.
Accordingly, there is a need in the art for a low-cost actuator which accurately provides an output force and a response time adequate for compensating for distortions in laser optics systems.